Donor natural killer cell allorecognition of missing self in haploidentical hematopoietic transplantation for acute myeloid leukemia: challenging its predictive value

Natural killer cells and tumor control

PURPOSE OF REVIEW

After hematopoietic cell transplantation (HCT) donor-derived natural killer (NK) cells kill tumor cells to prevent relapse and mediate other beneficial clinical effects including control of infections without inducing graft-vs.-host disease (GVHD). Understanding the determinants of NK cell alloreactivity and function will support improvements in the design of HCT and adoptive cellular therapies.

RECENT FINDINGS

Refinements to the model of NK cell education or licensing have been made which will inform strategies to develop functional alloreactive NK cells for therapeutic use. Differences in NK cell function have been shown to be dependent on the nature of the stimuli. Recent advances have been made in our understanding of the role of activating NK receptors on education and outcome after HCT. The use of adoptively transferred NK cells to treat hematopoietic malignancies has been expanding. New approaches to modulate target sensitivity to NK cell-mediated killing are under development.

SUMMARY

NK cells play an important role in the therapeutic efficacy of HCT, with effects on control of infections, GVHD, engraftment and relapse prevention. Recent advances in our understanding of NK cell biology will support improvements in our ability to exploit NK cells to treat cancer.

Infusion of allogeneic natural killer cells in a patient with acute myeloid leukemia in relapse after haploidentical hematopoietic stem cell transplantation

BACKGROUND

Allogeneic donor natural killer (NK)-cell infusion (NK-DLI) is a promising immunotherapy for patients with hematologic disorders.

CASE REPORT

This report describes the case of a patient who received a single haploidentical NK-DLI for a relapse of acute myeloid leukemia (AML) after haploidentical hematopoietic stem cell transplantation. He underwent a cytoreductive, immunosuppressive regimen before NK-DLI and received high-dose interleukin-2 in vivo for 8 weeks afterward.

RESULTS

No major adverse effect was observed. Prospective phenotypic and functional studies of the NK cells showed major expansion of infused NK cells and, more importantly, of the alloreactive KIR2DL1+KIR2DL2/DL3-NKG2A- subset, which reached 117×10(6) cells/L on Day +14 after NK-DLI, the greatest expansion of infused alloreactive NK cells reported so far. Infused NK cells conserved their lytic capacities against K562 target cells and primary AML-mismatched blasts.

CONCLUSION

We review the literature to clarify these data and to detail the indications for allogeneic NK-DLI, the criteria for determining the most suitable donor, the types of conditioning regimens, and the procedures for selecting and activating NK cells.

Elimination of the chemotherapy resistant subpopulation of 4T1 mouse breast cancer by haploidentical NK cells cures the vast majority of mice

Metastatic breast cancer is currently incurable despite initial responsiveness, assumingly due to the presence of chemoresistant subpopulations that can be characterized as label retaining cells (LRC). In the 4T1 mouse breast cancer model, we previously achieved cure after Cyclophosphamide and Total Body Irradiation (CY + TBI) followed by haploidentical bone marrow and spleen transplantation (BMSPLT). CY + TBI without transplantation induced only transient impaired tumor growth indicating a critical role of donor immune cells. As it remained unknown if the 4T1 model resembles human disease with respect to the presence of subpopulations of chemoresistant LRC, we now demonstrate this is indeed the case. Chemoresistance of 4T1 LRC was demonstrated by in vitro co-incubation of fluorescently labeled 4T1 cells in limiting dilution with cyclophosphamide, doxorubicin or cisplatinum, after which only LRC containing colonies remained. LRC also remain in vivo after treatment with CY + TBI. Succeeding experiments set up to identify the haploidentical effector cell responsible for cure and, therefore, for the elimination of chemoresistant LRC designate donor NK cells crucial for the anti-tumor effect. NK cell depletion of the haploidentical graft fully abrogated the anti-tumor effect. Increased disease-free survival retained after transplantation of haploidentical bone marrow and NK cell-enriched spleen cell grafts, even in the absence of donor T-cells or of donor bone marrow. Tumor growth analysis indicates the anti-tumor effect being immediate (days). Based on these data, it is worthwhile to explore alloreactive adoptive NK cell therapy as consolidation for patients with metastasized breast cancer.

Is there an impact of killer cell immunoglobulin-like receptors and KIR-ligand incompatibilities on outcomes after unrelated cord blood stem cell transplantation?

Donor killer cell immunoglobulin-like receptor (KIR) ligand incompatibility in the graft-versus-host direction is associated with decreased relapse incidence and improved disease-free survival after haploidentical and human leucocyte antigen (HLA)-mismatched unrelated, haematopoietic stem cell transplantation. However, review of all published studies of allogeneic HLA-matched or mismatched stem cell transplantation shows that the results on the relationship between donor-recipient KIR(-ligand) (in)compatibility and outcomes are highly variable, ranging from highly beneficial to detrimental. Reasons for these differences may include the methodology to determine KIR(-ligand) incompatibility, the disease distribution and the transplant protocol or donor type. Two retrospective studies on the effects of KIR-ligand incompatibility in unrelated cord blood transplantation (UCBT) for haematological malignancies have resulted in conflicting results. The Eurocord study showed a favourable effect of KIR-ligand mismatching on relapse incidence and leukaemia-free survival, whereas the Minneapolis study showed no effect on these end points and a detrimental effect on incidence of graft-versus-host disease (GvHD). In patients with non-malignant disorders, KIR-ligand (in)compatibility between donor and recipient was not associated with outcomes in a recent Eurocord analysis. Therefore, the role of natural killer (NK) cell alloreactivity in UCBT is far from clear. It is too early to use a donor-recipient KIR(-ligand) algorithm for selection of a cord blood donor.

Alternative donors hematopoietic stem cells transplantation for adults with acute myeloid leukemia: Umbilical cord blood or haploidentical donors?

Use of allogeneic transplantation for patients with acute myeloid leukemia (AML) depends mainly on the risk of the disease, and HLA matched donor availability. In patients with high-risk leukemia, in the absence of a HLA (human leukocyte antigen) matched donor, alternative donors such as unrelated umbilical cord blood (UCB) or haploidentical donor (haplo) have been currently used. Both strategies have important advantages such as shorter time to transplant, which is particularly relevant to patients requiring urgent transplantation, and tolerance of HLA mismatched graft that make possible that a donor can be found for virtually all patients. However, in spite of higher incidence of graft failure in UCB transplatation recipients and higher relapse incidence after haplo transplants, final outcomes seem to be comparable with HLA matched unrelated hematopoietic stem cell transplantation (bone marrow or peripheral blood). Therefore, the complexity of choosing the best alternative donor will depend on urgency of the transplantation, status and risk of the disease, donor criteria and center experience. Here we review the current status of UCBT and haplo transplants to treat adults with high-risk acute myeloid leukemia and we discuss the main issues associated with the use of both hematopoietic stem cell transplant approaches.

Haploidentical hematopoietic stem-cell transplantation in adults

Haploidentical hematopoietic stem-cell transplantation is an alternative transplant strategy for patients without an HLA-matched donor. Still, only half of patients who might benefit from transplantation are able to find an HLA-matched related or unrelated donor. Haploidentical donor is readily available for many patients in need of immediate stem-cell transplantation. Historical experience with haploidentical stem-cell transplantation has been characterised by a high rejection rate, graft-versus-host disease, and transplant-related mortality. Important advances have been made in this field during the last 20 years. Many drawbacks of haploidentical transplants such as graft failure and significant GVHD have been overcome due to the development of new extensive T cell depletion methods with mega dose stem-cell administration. However, prolonged immune deficiency and an increased relapse rate remain unresolved problems of T cell depletion. New approaches such as partial ex vivo or in vivo alloreactive T cell depletion and posttransplant cell therapy will allow to improve immune reconstitution in haploidentical transplants. Results of unmanipulated stem-cell transplantation with using ATG and combined immunosuppression in mismatched/haploidentical transplant setting are promising. This paper focuses on recent advances in haploidentical hematopoietic stem-cell transplantation for hematologic malignancies.

Expansion of CD56-negative, CD16-positive, KIR-expressing natural killer cells after T cell-depleted haploidentical hematopoietic stem cell transplantation

The main functions of natural killer (NK) cells are early protection against viruses or tumor cells and production of cytokines that regulate immune functions. The present study assessed the role of different NK subsets in exerting graft-versus-leukemia effects in recipients of human leukocyte antigen (HLA) haploidentical hematopoietic transplants and monitored for the first time CD3-/CD56- lymphocyte expansion. CD3-/CD56- cells expressed NK cell-associated molecules, such as CD16, NKp46, NKp30, CD244 (2B4), CD161, and killer cell immunoglobulin-like receptors. CD3-/CD56- cells further exhibited the classical functional characteristics of NK cells: cytolysis of target cells lacking HLA class I, antibody-dependent cellular cytotoxicity and cytokine production. These results demonstrate that CD56- NK cells are functional, recognize missing self and, like their CD56+ counterparts, may contribute to graft-versus-leukemia reactions.

Bridging innate NK cell functions with adaptive immunity

Killer Ig-like receptors (KIRs) are major human NK receptors displaying either inhibitory or activating functions which recognize allotypic determinants of HLA-class I molecules. Surprisingly, NK cell treatment with CpG-ODN (TLR9 ligands) results in selective down-modulation of KIR3DL2, its co-internalization with CpG-ODN and its translocation to TLR9-rich early endosomes. This novel KIR-associated function may offer clues to better understand the possible role of certain KIRs and also emphasizes the involvement of NK cells in the course of microbial infections. NK cells are involved not only in innate immune responses against viruses and tumors but also participate in the complex network of cell-to cell interaction that leads to the development of adaptive immune responses. In this context the interaction of NK cells with DC appears to play a crucial role in the acquisition of CCR7, a chemokine receptor that enables NK cells to migrate towards lymph nodes in response to CCL19 and/or CCL21. Analysis of NK cell clones revealed that KIR-mismatched but not KIR-matched NK cells acquire CCR7. These data have important implications in haploidentical haematopoietic stem cell transplantation (HSCT), in which KIR-mismatched NK cells may acquire the ability to migrate to secondary lymphoid compartments (SLCs), where they can kill recipient's antigen presenting cells (APCs) and T cells thus preventing graft versus host (and host vs. graft) reactions.

Human natural killer cells exhibit direct activity against Aspergillus fumigatus hyphae, but not against resting conidia

Because natural killer (NK) cells kill tumor cells and combat infections, there is growing interest in adoptively transferring NK cells to hematopoietic stem cell recipients. Unfortunately, in humans, the activity of NK cells against Aspergillus species, the major cause of invasive fungal infection in stem cell recipients, are poorly characterized. Our results show that unstimulated and interleukin-2 prestimulated human NK cells kill Aspergillus fumigatus hyphae but do not affect resting conidia. Killing is also induced by the supernatant of prestimulated NK cells and human perforin. The high levels of interferon-γ and granulocyte macrophage colony-stimulating factor produced by prestimulated NK cells are significantly reduced by Aspergillus, indicating an immunosuppressive effect of the fungus. Whereas Aspergillus hyphae activate NK cells, resting, and germinating, conidia and conidia of ΔrodA mutants lacking the hydrophobic surface layer do not. Our results suggest that adoptively transferred human NK cells may be a potential antifungal tool in the transplantation context.

Long-term follow-up of a pilot study using a chemotherapy-alone protocol for killer Ig-like receptor-ligand-mismatched haploidentical haematopoietic SCT

Advanced haematological malignancies are incurable without allogeneic haematopoietic SCT (HSCT). Many patients do not have a human leukocyte Ag (HLA)-matched donor; hence, haploidentical HSCT has been explored for some 20 years. Previous poor outcomes have improved recently with modifications, including the use of killer Ig-like receptor (KIR)-ligand-mismatched donors and highly T-cell-depleted megadose CD34+ stem cell infusions. Haploidentical HSCT was undertaken in 10 patients with heavily pretreated and advanced myeloid malignancies. Patient/donor pairs were KIR-ligand mismatched (GVL direction). Conditioning regimen was ATG, melphalan, fludarabine and thiotepa. G-CSF-mobilized PBSCs were CD34+ cell selected. No post transplant immunosuppression was given. Two patients died early; all others had sustained engraftment. Natural killer cell recovery, often to supranormal levels, occurred early, whereas CD4+ T-cell recovery was delayed. Acute GVHD occurred in three of eight (30%) patients, and chronic GVHD occurred in three of six (50%) evaluable patients. No infections with Candida or Aspergillus developed in seven patients receiving caspofungin prophylaxis. Three of 10 (30%) patients were alive and disease free at 10.1, 6 and 5.4 years post transplant (Karnovsky scores of 100%). In this heavily pretreated cohort with very advanced myeloid malignancies, KIR-ligand-mismatched haploidentical HSCT cured a significant proportion of patients.

Post-transplant immune reconstitution after unrelated allogeneic stem cell transplant in patients with acute myeloid leukemia

We evaluated immune recovery in 67 patients with acute myeloid leukemia (AML) with a median age of 40 years (4-69) following allo-SCT after reduced (n = 35) or myeloablative (n = 32) conditioning. The following lymphocyte populations were determined on days +30, +90, +180, +270, and +365 by flow associated cell sorting: CD3+, CD3+CD4+, CD3+CD8+, CD3+CD4+/CD3+CD8+ ratio, CD3-CD56+, and CD19+ cells. Peripheral blast count >5% was related to lower number of CD3+CD4+ (day +30) and NK cells (day +180; p = 0.02). Intensity of conditioning did not have any significant impact on the kinetics of immune recovery. Patients with normal CD3+CD4+/CD3+CD8+ ratio (day +30) and NK cell count (day +90; p <0.05) experienced better survival than those with decreased parameters. Post-transplant sepsis/severe infections impaired CD3+CD8+ (day +90; p = 0.015) and CD19+ (day +90; p = 0.02) recovery. Relapse in patients following allo-SCT showed an association with decreased numbers of CD19+ (day +270) and NK cells (day +365). Acute GvHD (II-IV) was accompanied by reduced CD19+ and CD3+CD4+ cells. Thus, the evaluation of post-transplant immune reconstitution in patients with AML might improve risk stratification concerning either relapse or TRM and remains to be further explored.

Human bone marrow stromal cells and skin fibroblasts inhibit natural killer cell proliferation and cytotoxic activity

Mesenchymal stromal cells (MSCs) are potent immunomodulators that have successfully been used to circumvent various types of inflammations, including steroid-resistant graft-versus-host disease. Although initially believed to be restricted to multipotent MSCs, this immunoregulatory function is shared with differentiated cells from the mesenchymal lineage such as skin fibroblasts (SFs). Mesenchymal cell-induced immunoregulation is so potent that it may allow the reactivation of dormant malignancies, a fact that would preclude using such cells as therapeutic agents. Because NK cells are pivotal effectors controlling tumor cell containment we investigated the effect of allogenic MSCs and SFs on NK cell function in vitro. When NK cells were incubated with IL-15 and MSCs or SFs for 6 days, their proliferation and cytotoxic activity were significantly decreased compared to NK cells cultured with IL-15 alone or with human venous endothelial cells. Cytotoxic activity inhibition reached 86% when assayed on MHC-I(+) allogenic primary hematopoietic blasts, and was associated with a significant decrease in cytolytic granule exocytosis and in perforin release. Stromal cell-mediated inhibition was effective only if cell-cell proximity was long lasting: when NK cells were activated with IL-15 in the absence of MSCs and assayed for cytotoxicity in their presence no inhibition occurred. MSC inhibition was ultimately mediated by a soluble factor generated upon incubation with NK cells activated by IL-15 or IL-2. The indoleamine 2,3 dioxygenase was activated in MSCs and SFs because L-kynurenine was detected in inhibitory supernatants, but its blockade did not restore NK cell functions. The profound inhibition of cytotoxic activity directed against allogenic hematopoietic blasts exerted by MSCs and SFs on NK cells may be a concern. Should this occur in vivo it may induce the inability of NK cells to control residual or dormant malignant diseases after infusion of therapeutic MSCs.

Haploidentical transplantation for leukemia

Hematopoietic stem cell transplantation from human leukocyte antigen (HLA)-haploidentical family members offers a potential cure for patients in need of allogeneic immunotherapy who have no immediate access to an HLA-matched donor. The use of ex vivo T-cell-depleted stem cells combined with immuno-myeloablative conditioning has enabled durable donor engraftment with a low incidence of acute graft-versus-host disease despite the HLA disparity. Moreover, additional transplant techniques involving in vivo T-cell depletion and reduced-intensity conditioning have further minimized the risks. However, a major drawback is delayed immune reconstitution leading to infections and high relapse rates, prompting significant research efforts focused on improving recovery in the post-transplant period. Infusions with donor lymphocytes are common, though newer manipulations with a focus on donor natural killer cells hold great promise, as do other modified donor T-cell infusions. Success of these new procedures will make haploidentical transplants safer and more effective, further broadening its appeal.

NKT cells, Treg, and their interactions in bone marrow transplantation

Bone marrow transplantation (BMT) is a potentially curative treatment for patients with leukemia and lymphoma. Tumor eradication is promoted by the anti-tumor activity of donor T cells contained in the transplant; however, donor T cells also mediate the serious side effect of graft-versus-host disease (GVHD). Separation of GVHD from graft anti-tumor activity is an important goal of research in improving transplant outcome. One approach is to take advantage of the immunomodulatory activity of regulatory NKT cells and CD4(+)CD25(+) Treg of host and/or donor origin. Both host and donor NKT cells and donor Treg are able to prevent GVHD in murine models. In this review, we summarize the mechanisms of NKT cell- and Treg-mediated protection against GVHD in mice while maintaining graft anti-tumor activity. In addition, we also examine the interactions between NKT cells and Treg in the context of BMT, and integrate the data from murine experimental models with the observations made in humans.

Alloreactive natural killer cells in hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) for leukemia can play a major role in reducing the risk of relapse by inducing a graft versus leukemia (GVL) effect. Here, we review the effectiveness of mismatching inhibitory killer-cell-immunoglobulin-like receptors (KIR) on donor natural killer (NK) cells as a mechanism for GVL. We review the range of KIR and the importance of T cell and NK cell content of the graft, together with considerations of the graft source. Further understanding of conditioning and mechanisms to reduce graft versus host disease (GVHD) will improve our ability to manipulate NK cells in HSCT.

Haploidentical hematopoietic stem cell transplantation to adults with hematologic malignancies: analysis of 66 cases at a single Japanese center

Sixty-six adult patients with hematologic malignancies underwent haploidentical hematopoietic stem cell transplantation (haplo-HSCT) without T cell depletion. The patients were preconditioned with a reduced intensity regimen, and tacrolimus was used for graft-versus-host disease (GVHD) prophylaxis. Successful engraftment occurred in 60 patients (90.1%) and graft rejection in only 4 patients (6.1%). Among the 60 engrafted patients, only 5 developed severe (grade III or IV) acute GVHD. Twenty patients, including 19 relapse-free patients were alive at a median follow-up of 48 months (range 6-77 months). The overall survival (OS) at 6 years was 29.3%. The OS of 45 patients < 60 years of age was 43.6%, which was superior to that of 21 patients who were 60 years of age and older (9.5%) (P < 0.01). The OS of 11 patients from human leukocyte antigen (HLA) 1 locus-mismatched donors (63.6%) was higher than that of 28 patients from HLA 3 loci-mismatched donors (12.5%) (P < 0.01). Organ injury and infection were the main causes of mortality. Notably, immunosuppressive therapy could be successfully stopped in 9 patients transplanted from HLA 2 or 3 loci-mismatched donors with a median duration of 45 months (range 5-71 months). These data suggest that haplo-HSCT is a promising treatment for patients who need urgent allogeneic transplantation but lack HLA-identical family donors.

Natural killer cell-enriched donor lymphocyte infusions from A 3-6/6 HLA matched family member following nonmyeloablative allogeneic stem cell transplantation

Infusing natural killer (NK) cells following transplantation may allow less infections and relapse with little risk of acute graft-versus-host disease (aGVHD). We delivered 51 total NK cell-enriched donor lymphocyte infusions (DLIs) to 30 patients following a 3-6/6 HLA matched T cell-depleted nonmyeloablative allogeneic transplant. The primary endpoint of this study was feasibility and safety. Eight weeks following transplantation, donor NK cell-enriched DLIs were processed using a CD56(+) selecting column with up to 3 fresh infusions allowed. Toxicity, relapse, and survival were monitored. T cell phenotype, NK cell functional recovery, and KIR typing were assessed for association with outcomes. Fourteen matched and 16 mismatched transplanted patients received a total of 51 NK cell-enriched DLIs. Selection resulted in 96% (standard deviation [SD] 8%) purity and 83% (SD 21%) yield in the matched setting and 97% (SD 3%) purity and 77% (SD 24%) yield in the mismatched setting. The median number of CD3(-) CD56(+) NK cells infused was 10.6 (SD 7.91) x 10(6) cells/kg and 9.21 (SD 5.6) x 10(6) cells/kg, respectively. The median number of contaminating CD3(+)CD56(-) T cells infused was .53 (1.1) x 10(6) and .27 (.78) x 10(6) in the matched and mismatched setting, respectively. Only 1 patient each in the matched (n = 14) or mismatched (n = 16) setting experienced severe aGVHD with little other toxicity attributable to the infusions. Long-term responders with multiple NK cell-enriched infusions and improved T cell phenotypic recovery had improved duration of responses (p = .0045) and overall survival (OS) (P = .0058). A 1-step, high-yield process is feasible, and results in high doses of NK cells infused with little toxicity. NK cell-enriched DLIs result in improved immune recovery and outcomes for some. Future studies must assess whether the improved outcomes are the direct result of the high doses and improved NK cell function or other aspects of immune recovery.

Extending killer Ig-like receptor function: from HLA class I recognition to sensors of microbial products

Killer Ig-like receptors (KIRs) are human natural killer (NK) receptors that recognize allotypic determinants of human leukocyte antigen (HLA) class I. Inhibitory KIRs discriminate normal cells from tumour or virus-infected cells that have lost or reduced HLA class I expression. Donor NK cell "alloeffector" responses are exploited in haploidentical haematopoietic stem cell transplantation to treat leukaemia. NK cells also express several toll-like receptors (TLRs) that increase NK cell cytotoxicity and cytokine release in response to ligands. Surprisingly, KIR3DL2 binds the TLR ligand CpG-oligodexynucleotides, and together, they are co-internalized and translocated to TLR9-rich early endosomes. This novel KIR-associated function offers clues to understanding the NK cell response to microbial infection, and extends the role played by KIRs in immune defence.

Results and factors influencing outcome after fully haploidentical hematopoietic stem cell transplantation in children with very high-risk acute lymphoblastic leukemia: impact of center size: an analysis on behalf of the Acute Leukemia and Pediatric Disease Working Parties of the European Blood and Marrow Transplant group

T cell–depleted haploidentical hematopoietic stem cell transplantation (haploHSCT) is an option to treat children with very high-risk acute lymphoblastic leukemia (ALL) lacking an HLA-identical donor. We analyzed 127 children with ALL who underwent haploHSCT in first (n = 22), second (n = 48), or third (n = 32), complete remission or in relapse (n = 25). The 5-year leukemia-free survival (LFS) was 30%, 34%, 22%, and 0%, respectively. A risk-factor analysis was performed for patients who underwent transplantation in remission (n = 102). Five-year nonrelapse mortality (NRM), relapse incidence (RI), and LFS were 37%, 36%, and 27%, respectively. A trend of improved LFS rate and decreased RI was observed for children given a graft with higher number of CD34+ cells (adjusted P = .09 and P = .07, respectively). In a multivariate analysis, haploHSCT performed in larger centers (performing ≥ 231 allotransplantations in the studied period) was associated with improved LFS rate and decreased RI (adjusted P = .01 and P = .04, respectively), adjusting for different patient-, disease-, and transplant-related factors such as number of previous autotransplantations, cytomegalovirus serology status, type of T-cell depletion, and use of total body irradiation and antithymocyte globulin. In conclusion, higher CD34+ cell dose and better patient selection may improve outcomes of children with ALL who undergo a haploHSCT. Transplant centers initiating programs on haploHSCT for children may collaborate with more experienced centers.

Biology and clinical effects of natural killer cells in allogeneic transplantation

PURPOSE OF REVIEW

Following allogeneic hematopoietic cell transplantation, donor-derived natural killer (NK) cells target recipient hematopoietic cells, resulting in an antileukemia effect and a lower incidence of graft rejection. NK cells do not mediate and may diminish graft versus host disease. Here we review the determinants of NK cell alloreactivity and their implications for adoptive NK cell therapy.

RECENT FINDINGS

NK cell alloreactivity has been defined by the absence of recipient MHC class I ligands for donor inhibitory killer immunoglobulin-like receptor (KIR) receptors, as predicted by a number of algorithms. Recently, the role of activating NK receptors and their cognate ligands has received more attention. The beneficial clinical effect of NK-cell alloreactivity has not been uniformly demonstrated, likely reflecting differences in conditioning regimens, graft components and posttransplant immune suppression. Investigations of NK cell phenotype and function after transplantation have helped demonstrate which NK cell subsets mediate the graft versus leukemia effect. These advances have proceeded in parallel with increasing facility in GMP-grade bulk purification and administration of NK cell preparations.

SUMMARY

NK cells are a heterogeneous population of lymphocytes with diverse patterns of target-cell recognition and effector function. Further clinical and functional correlations will help maximize their potential for clinical benefit.

Transplant outcomes in acute leukemia. II

Currently, it is possible to find a hematopoietic stem cell (HSC) donor for virtually all patients with acute leukemia who have an indication to receive an allogeneic hematopoietic stem cell transplant (HSCT) and lack a human leukocyte antigen (HLA)-identical sibling or a well-matched HLA unrelated donor (URD). According to the ethnicity of the patients and the donor registry, approximately 25% to 60% of patients will not find an 8/8 HLA-matched unrelated donor. Other alternative donors, such as HLA-mismatched related donor or unrelated donor umbilical cord blood (UCB), have emerged to solve the lack of a sibling or well-matched URD. In the haploidentical HSCT setting, new techniques of T-cell depletion, new approaches using combinations of immunosuppressive drugs or different conditioning regimens, and developments on immunotherapy have focused attention on this option. Therefore, any physician has to carefully evaluate, for each patient in need of an allograft, all of the possible alternatives in order to choose the best HSC donor, taking into account type of disease to be transplanted, urgency of transplantation, donor characteristics, and center experience. This review evaluates the current status of haploidentical HSCT in acute leukemia, its advantages and remaining limitations compared to other stem cell sources, and how these data may be used in the development of donor selection algorithms.

Unmanipulated HLA-mismatched/haploidentical blood and marrow hematopoietic stem cell transplantation

Extensive ex vivo T cell-depleted or unmanipulated haploidentical transplantation provides benefits of rapid and near universal donor availability for patients without HLA-identical sibling donors or those who urgently need transplant. However, CD34 selected haplotype mismatched transplantation was limited by delayed immune reconstitution (IR), although this protocol has now been an acceptable approach. Recently, Peking University researchers developed a novel approach to HLA-mismatched/haploidentical blood and marrow transplantation without in vitro T cell depletion (GIAC protocol). This review summarizes transplant outcomes, and factors correlating with transplant outcomes following the GIAC protocol. Moreover, future challenges in improving posttransplant IR and finding the best approach reducing the incidence and severity of GVHD, whereas preserving graft-versus-leukemia effect to prevent the recurrence of underlying malignancy, are also discussed.

Natural killer-cell KIR repertoire reconstitution after haploidentical SCT

We studied killer-cell Ig-like receptor (KIR)/natural killer (NK)-cell group-2-Ag repertoires on donor-derived NK cells in 28 patients after haploidentical SCT in the first 6 months after SCT and correlated results with EFS. The reconstitution hierarchy of potentially alloreactive, single KIR+ NK cells was the following: HLA-C1 binding>HLA-Bw4 binding>HLA-C2 binding. The differences in reconstitution kinetics of the three potentially alloreactive NK cell subsets prompted an updated analysis of EFS in AML patients transplanted from haploidentical donors in our center. This analysis showed that in haploidentical transplantation for AML, HLA-C group 1 mismatching in the graft vs host direction not only provides a survival advantage over non-NK-alloreactive (KIR ligand-matched) transplants (5-year EFS 67±10% vs 17±5%) but, indeed, also provides the best EFS compared with C2 (35±10%) or Bw4 KIR ligand mismatches (44±17%). In conclusion, we show that the kinetics with which single KIR-expressing NK cells are generated after haploidentical SCT differ between individual KIR receptors and seem to influence survival after haploidentical SCT.

NCI First International Workshop on The Biology, Prevention, and Treatment of Relapse After Allogeneic Hematopoietic Stem Cell Transplantation: Report from the Committee on the Biology Underlying Recurrence of Malignant Disease following Allogeneic HSCT: Graft-versus-Tumor/Leukemia Reaction

The success of allogeneic hematopoietic stem cell transplantation (HSCT) depends on the infusion of benign stem cells as well as lymphocytes capable of participating in a graft-versus-tumor/leukemia (GVL) reaction. Clinical proof of concept is derived from studies showing increased relapse after the infusion of lymphocyte depleted hematopoietic grafts as well as the therapeutic efficacy of donor lymphocyte infusions without chemotherapy to treat relapse in some diseases. Despite this knowledge, relapse after allogeneic HSCT is common with rates approaching 40% in those with high-risk disease. In this review, we cover the basic biology and potential application to exploit adaptive T cell responses, minor histocompatibility antigens, contraction and suppression mechanisms that hinder immune responses, adaptive B cell responses and innate NK cell responses, all orchestrated in a GVL reaction. Optimal strategies to precisely balance immune responses to favor GVL without harmful graft-versus-host disease (GVHD) are needed to protect against relapse, treat persistent disease and improve disease-free survival after HSCT.

Expression of natural killer receptors in T- and NK-cells: comparison of healthy individuals, patients with prior stem cell transplant, and patients undergoing chemotherapy

We studied the expression of natural killer receptors (NKRs) on peripheral blood cytotoxic T lymphocytes and NK cells in patients who underwent an allogeneic stem cell transplant (SCT), and compared these findings with results from healthy individuals (CTRL) and patients undergoing chemotherapy (CHEMO), respectively. Peripheral blood mononuclear cells were analyzed by flow cytometry with antibodies against the NKRs CD158a, CD158b, CD158e (known as killer immunoglobulin-like receptors, KIRs), and CD94. Expression of NKRs was evaluated separately in CD56+, CD57+, and CD56/CD57 (double +) subsets of T and NK cells. We found mainly differences in CD158a and CD94 expression between the three cohorts, with the SCT and CHEMO groups usually showing similar changes, when compared to the CTRL population. None of the patients with SCT or CHEMO demonstrated patterns of restricted NKR expression. Our results provide a comprehensive overview of KIR and CD94 expression in T and NK cells following SCT and chemotherapy.

Cancer immunotherapy: will expanding knowledge lead to success in pediatric oncology?

The past 25 years have seen an increase in our understanding of immunology and further expansion in the clinical use of immunotherapeutic modalities. How immunotherapy will be integrated with chemotherapy, radiation, and surgery remains to be established. Although there have been successes in the field of immunotherapy, they have been inconsistent, and it is hoped that increased understanding of the basic principles of immunology will improve the consistency of beneficial effects. In this article, we briefly provide a general overview of our current understanding of the immune system, with a focus on concepts in tumor immunology, followed by a discussion of how these concepts are being used in the clinic.

Immunotherapy in the context of hematopoietic stem cell transplantation: the emerging role of natural killer cells and mesenchymal stromal cells

Immunotherapy in the context of hematopoietic stem cell transplantation has been dominated for many years by T-cell- and dendritic-cell-based treatment modalities. During the last decade, insight into the biology of natural killer (NK) cells and mesenchymal stromal cells (MSC) has rapidly increased and resulted in NK- and MSC-based therapeutic strategies in clinical practice. This article reviews current knowledge of the biology and clinical aspects of NK cells and MSC.

Nonmyeloablative HLA-haploidentical bone marrow transplantation with high-dose posttransplantation cyclophosphamide: effect of HLA disparity on outcome

Although some reports have found an association between increasing HLA disparity between donor and recipient and fewer relapses after allogeneic blood or marrow transplantation (BMT), this potential benefit has been offset by more graft-versus-host disease (GVHD) and nonrelapse mortality (NRM). However, the type of GVHD prophylaxis might influence the balance between GVHD toxicity and relapse. The present study analyzed the impact of greater HLA disparity on outcomes of a specific platform for nonmyeloablative (NMA), HLA-haploidentical transplantation. A retrospective analysis was performed of 185 patients with hematologic malignancies enrolled in 3 similar trials of NMA, related donor, haploidentical BMT incorporating high-dose posttransplantation cyclophosphamide for GVHD prophylaxis. No significant association was found between the number of HLA mismatches (HLA-A, -B, -Cw, and -DRB1 combined) and risk of acute grade II-IV GVHD (hazard ratio [HR] = 0.89; P = .68 for 3-4 vs fewer antigen mismatches). More mismatching also had no detrimental effect on event-free survival (on multivariate analysis, HR = 0.60, P = .03 for 3-4 vs fewer antigen mismatches and HR = 0.55, P = .03 for 3-4 vs fewer allele mismatches). Thus, greater HLA disparity does not appear to worsen overall outcome after NMA haploidentical BMT with high-dose posttransplantation cyclophosphamide.

The impact of KIR2DS4 alleles and the expression of KIR in the development of acute GVHD after unrelated allogeneic hematopoietic SCT

The role of killer Ig-like receptors (KIR) in SCT was analyzed. A total of 75 Chinese patients were transplanted with T-depleted hematopoietic stem cells from unrelated donors. Among the 75 donor-recipient pairs, 60 were HLA 10/10 matched and 15 had some mismatches at HLA-C. Transplants from KIR haplotype B/x group donors showed significantly higher overall survival rates compared with those from KIR haplotype A/A donors (relative risk (RR) 3.1 (95% confidence interval (CI) 1.1-8.6), P=0.007). In the haplotype A/A group, a higher risk of acute GVHD (aGVHD) (RR 9.0 (95% CI 1.2-66.9), P=0.01), especially grade III-IV aGVHD (P=0.006), was observed when the donor was homozygous for the full-length expressed KIR2DS4*00101 allele. Real-time PCR showed that a high expression of inhibitory KIR (2DL1 and 3DL1) in the early stages (<90 days) after transplantation correlated with the development of aGVHD (z=2.558, P=0.011). Our findings indicated a significant association of full-length KIR2DS4 or KIR2DL1/3DL1 expression with the occurrence of aGVHD. In aggregate these results suggested that combining KIR and HLA genotyping could help in the selection of transplant donors and improve the outcome of transplantation. Dynamic detection of KIR2DL1/3DL1 expression would be beneficial for prediction of aGVHD after transplantation.

Expansion of NKG2A-LIR1- natural killer cells in HLA-matched, killer cell immunoglobulin-like receptors/HLA-ligand mismatched patients following hematopoietic cell transplantation

The prognosis after hematopoietic cell transplantation (HCT) for the treatment of leukemia or lymphoma in humans is influenced by donor-derived natural killer (NK) cells, which enhance the graft-versus-leukemia (GVL) effect. Such alloreactive killer cells can be generated in vivo after HCT if the donor expresses killer cell immunoglobulin-like receptors (KIRs), such as KIR2DL1, KIR2DL2/3, or KIR3DL1, for which the recipient lacks HLA class I ligands. We studied effector cells from 22 KIR/HLA-ligand mismatched and 14 KIR/HLA-ligand matched, primarily HLA-matched patient-donor pairs after allogeneic HCT. A novel 8-color flow cytometry panel allowed us to characterize effector-cell populations without "broadly reactive" inhibitory receptors such as CD94/NKG2A or LIR1. The numbers of such NKG2A(-) LIR1(-) NK cells increased following HCT in patients transplanted by KIR/HLA-ligand mismatched grafts, compared to KIR/HLA-ligand matched grafts, and in patients transplanted from donors of the A/B, compared to A/A, KIR haplotypes. NKG2A(-)LIR1(-) NK cells expressing only those inhibitory KIRs for which the patient had no HLA class I ligands could be stimulated by HLA class I-deficient cells to express CD107a. Thus, NKG2A(-)LIR1(-) NK cells may be important GVL effector cells following HCT, even in patients transplanted from HLA-matched donors.

Natural killer cells: tolerance to self and innate immunity to viral infection and malignancy

Natural killer (NK) cells are lymphocytes whose ability to identify and kill virally infected and malignant cells while sparing normal cells was poorly understood until the late 1980’s and the introduction of the “missing self’ hypothesis. According to this hypothesis, downregulation of major histocompatibility complex (MHC) class I molecules during viral infection or malignant transformation triggers NK activation (1 ). Since this hypothesis was first proposed, much has been learned about NK cell surface receptors, their role in the molecular basis of missing-self recognition, and the mechanisms underlying NK cell tolerance. In this review, we will discuss these mechanisms, as well as their relevance to viral infection and tumor immunity and stem cell transplantation.

Impaired antigen presentation in neoplasia: basic mechanisms and implications for acute myeloid leukemia

During onset, treatment and progression of acute myeloid leukemia (AML), inadequate immune responses against certain myeloid leukemic blasts might be associated with the occurrence of minimal residual disease and subsequent relapse. Several studies on this subject have demonstrated that, in general, solid tumor cells are able to avoid CD8+ cytotoxic T-cell recognition by downregulating HLA class I-restricted presentation of tumor-associated antigens. In tumor cells that can express HLA class II molecules, such as myeloid leukemic blasts, abnormalities in the processing pathways of endogenous antigens could also result in impaired HLA class II-restricted tumor-associated antigen presentation to CD4+ T helper cells. More insight into impaired tumor-associated antigen presentation by myeloid leukemic blasts could explain their escape from immune recognition and might be crucial for selecting appropriate strategies to improve whole-cell or dendritic cell-based tumor vaccine efficacy in the treatment of AML patients.

Cellular immunotherapy of cancer

Standard therapies for many common cancers remain toxic and are often ineffective. Cellular immunotherapy has the potential to be a highly targeted alternative, with low toxicity to normal tissues but a high capacity to eradicate tumor. In this chapter we describe approaches that generate cellular therapies using active immunization with cells, proteins, peptides, or nucleic acids, as well as efforts that use adoptive transfer of effector cells that directly target antigens on malignant cells. Many of these approaches are proving successful in hematologic malignancy and in melanoma. In this chapter we discuss the advantages and limitations of each and how over the next decade investigators will attempt to broaden their reach, increase their efficacy, and simplify their application.

Natural killer cells and cancer: regulation by the killer cell Ig-like receptors (KIR)

Natural killer (NK) cells are innate immune effector cells that make up approximately 10-15% of the peripheral blood lymphocytes in humans and are primarily involved in immunosurveillance to eliminate transformed and virally-infected cells. They were originally defined by their ability to spontaneously eliminate rare cells lacking expression of class I major histocompatibility complex (MHC-I) self molecules, which is commonly referred to as "missing self" recognition. The molecular basis for missing self recognition emerges from the expression of MHC-I-specific inhibitory receptors on the NK cell surface that tolerize NK cells toward normal MHC-I-expressing cells. By lacking inhibitory receptor ligands, tumor cells or virus-infected cells that have down-modulated surface MHC-I expression become susceptible to attack by NK cells. Killer cell Ig-like receptors (KIR; CD158) constitute a family of MHC-I binding receptors that plays a major role in regulating the activation thresholds of NK cells and some T cells in humans. Here, we review the multiple levels of KIR diversity that contribute to the generation of a highly varied NK cell repertoire and explain how this diversity can influence susceptibility to a variety of diseases, including cancer. We further describe strategies by which KIR can be manipulated therapeutically to treat cancer, through the exploitation of KIR/MHC-I ligand mismatch to potentiate hematopoietic stem cell transplantation and the use of KIR blockade to enhance tumor cell killing.

CD3+/CD19+-depleted grafts in HLA-matched allogeneic peripheral blood stem cell transplantation lead to early NK cell cytolytic responses and reduced inhibitory activity of NKG2A

Natural killer (NK) cells have an important function in the anti-tumor response early after stem cell transplantation (SCT). As part of a prospective randomized phase III study, directly comparing the use of CD3(+)/CD19(+)-depleted peripheral blood stem cell (PBSC) harvests with CD34(+)-selected PBSC harvests in allogeneic human leukocyte antigen-matched SCT, we here show that the use of CD3(+)/CD19(+)-depleted PBSC grafts leads to early NK cell repopulation and reconstitution of the CD56(dim) and CD56(bright) NK cell subsets, with concomitant high cytolytic capacity. In the CD34 group, this process took significantly longer. Moreover, in the CD3/19 group after reconstitution, a higher percentage of killer immunoglobulin-like receptor-positive NK cells was found. Although similar percentages of CD94-positive NK cells were found in both groups, in the CD34 group, almost all expressed the inhibitory CD94:NKG2A complex, whereas in the CD3/19 group, the inhibitory CD94:NKG2A and the activating CD94:NKG2C complex were equally distributed. This preferential development of NKG2C-expressing NK cells in the CD3/19 group was paralleled by a loss of NKG2A-mediated inhibition of NK cell degranulation. These results show that the use of CD3(+)/CD19(+)-depleted grafts facilitates strong NK cell cytolytic responses directly after SCT, and the rapid emergence of an NK cell receptor phenotype that is more prone to activation.

Improved survival with inhibitory killer immunoglobulin receptor (KIR) gene mismatches and KIR haplotype B donors after nonmyeloablative, HLA-haploidentical bone marrow transplantation

Natural killer (NK) cell alloreactivity, which may contribute to the graft-versus-leukemia (GVL) effect of allogeneic hematopoietic stem cell transplantation (HSCT), is influenced by the interaction of killer-cell immunoglobulin-like receptors (KIRs) on donor NK cells and their ligands, human leukocyte antigen (HLA) class I molecules on recipient antigen-presenting cells (APCs). Distinct models to predict NK cell alloreactivity differ in their incorporation of information from typing of recipient and donor KIR and HLA gene loci, which exist on different autosomes and are inherited independently as haplotypes. Individuals may differ in the inheritance of the 2 KIR haplotypes, A and B, or in the expression of individual KIR genes. Here, we examined the effect of KIR and HLA genotype, in both the recipient and donor, on the outcome of 86 patients with advanced hematologic malignancies who received nonmyeloablative (NMA), HLA-haploidentical HSCT with high-dose, posttransplantation cyclophosphamide (Cy). Compared to recipients of bone marrow (BM) from donors with identical KIR gene content, recipients of inhibitory KIR (iKIR) gene-mismatched BM had an improved overall survival (OS) (hazard ratio [HR]=0.37; confidence interval [CI]: 0.21-0.63; P=.0003), event-free survival (EFS) (HR=0.51; CI: 0.31-0.84; P=.01), and relapse rate (cause-specific HR, SDHR=0.53; CI: 0.31-0.93; P=.025). Patients homozygous for the KIR "A" haplotype, which encodes only 1 activating KIR, had an improved OS (HR=0.30; CI: 0.13-10.69; P=.004), EFS (HR=0.47; CI: 0.22-1.00; P=.05), and nonrelapse mortality (NRM; cause-specific HR=0.13; CI: 0.017-0.968; P=.046) if their donor expressed at least 1 KIR B haplotype that encodes several activating KIRs. Models that incorporated information from recipient HLA typing, with or without donor HLA typing, were not predictive of outcome in this patient cohort. Thus, NMA conditioning and T cell-replete, HLA-haploidentical HSCTs involving iKIR gene mismatches between donor and recipient, or KIR haplotype AA recipients of BM from KIR Bx donors, were associated with lower relapse and NRM and improved OS and EFS. These findings suggest that selection of donors based upon inhibitory KIR gene or haplotype incompatibility may be warranted.

Regulatory allospecific NK cell function is differentially associated with HLA C allotypes

Major histocompatibility complex I (MHC I) molecules 'silence' natural killer (NK) cell activity. Conversely, NK cell activity is triggered through cells lacking expression of autologous MHC I. Unexpectedly we found that a subset of NK cells is activated rather than silenced when interacting with cells expressing normal levels of autologous MHC I. Instead of inducing an inflammatory phenotype, however, activation led to the secretion of the regulatory cytokines TGF-beta and IL-10. Importantly, in vitro models of allogeneic interactions showed that targets co-expressing HLA C1 and C2 epitopes best supported, or even enhanced, this cell-contact-mediated regulatory NK cell function. Together, these data ascribe a novel pattern of reactivity to NK cells, with potential implications both in autologous and allogeneic systems.

Activating and inhibitory killer immunoglobulin-like receptors (KIR) in haploidentical haemopoietic stem cell transplantation to cure high-risk leukaemias

A number of experimental studies have shown that natural killer (NK) cells can eliminate cancer cells and the mechanisms involved in this effect have been uncovered during the last two decades. Clinical data from haploidentical haematopoietic stem cell transplantation (haplo-HSCT) revealed that NK cells were responsible for remarkably favourable effects in both adult and paediatric high-risk leukaemias. NK receptors specific for major histocompatibility complex (MHC) class I molecules, including killer immunoglobulin (Ig)-like receptors (KIR) and CD94/NKG2A, play a major role in the anti-leukaemia effect (mediating either inhibitory or activating signals). Haplo- HSCT requires a heavy conditioning regimen for the patient and the use of large numbers of T cell-depleted HSC to be grafted. After transplantation, natural killer cells develop from HSC shortly after engraftment and may include 'alloreactive' NK cells that kill leukaemic cells and prevent graft-versus-host disease (GvHD). Alloreactive NK cells are characterized by the expression of KIR that are not engaged by any of the human leucocyte antigen (HLA) class I alleles expressed by the patient. Their generation is dependent upon the existence of a KIR/HLA class I mismatch between donor and recipient. Novel important information on the function and specificity of different KIR has been obtained recently by the analysis of donor-derived alloreactive NK cells in a cohort of paediatric patients given haplo-HSCT to cure acute, high-risk leukaemias.